Atari 400 Technical Reference Manual page 129

AUXILIARY INFORMATION -- DAUX1/DAUX2 C030A/030BJ
The handler sets these 2-bytes. The SIO includes them in the bus
command frame; they have device-specific meanings.
Functions Supported
SIO does not examine the COMMAND byte it sends to the device,
because all bus transactions are expected to conform to a
universal protocol. The protocol includes three forms, stated
below (as seen from the computer):
Send command frame.
Send command frame and send data frame.
Send command frame and receive data frame.
The values of the Wand R bits in the status byte select the
command form.
Error Handling
SIO handles most of the serial bus errors for the handler,
as indicated below:
Bus timeout -- SIO provides a uniform command frame and data
frame ACK byte timeout of 1/60 of a second - 0 1 + 1/60.
The handler specifies the maximum COMPLETE byte timeout
value in DTIMLO.
Bus errors -- SIa detects and reports UART overrun and
framing errors. The sensing of these errors in any received
byte will cause the entire associated Trame to be considered
bad.
Data frame checksum error -- SIO validates the checksum on
all received data frames and generates a checksum Tor all
transmitted frames.
Invalid response from device -- In addition to the error
conditions stated above, SIO checks that the ACK and
COMPLETE responses are proper (ACK
=
$41 and COMPLETE
=
$43). ACK stands for acknowledge.
Bus operation retries -- SIO will attempt one complete command
retry if the first attempt is not error free, where a complete
command try consists of up to 14 attempts to send <and
acknowledge) a command frame. followed by a single attempt to
OPERATING SYSTEM C016555 -- Section 9
144